Much of this debate kicked off because the said Nature paper advances a hypothetical scenario for an abrupt Arctic methane release over either a decade or several decades of about 50 gigatonnes (Gt), and argues specifically that such a scenario is "likely." My own attempt to understand the literature convinced me that the scenario should be viewed as a serious possibility.

Tobis on the other hand is the latest amongst several scientists offering scathing criticisms of that scenario, which in his own words is "as close to impossible as anything in earth science; actual geophysics refutes it."He begins with my first point, 1. The 50 Gigatonne decadal methane pulse scenario was posited by four Arctic specialists, and is considered plausible by Met Office scientists.

"Arctic thawing may release in excess of 50 GT of C [Carbon], a very serious matter... But Ahmed refers to the paper in support of a very different assertion, that 50 GT of methane would be released... But the paper to which he points says nothing of the sort. I conclude that he doesn't really know what he is talking about. Specifically he has already shown that he is confused about the distinction between methane releases and CO2 releases."

However, the carbon release scenarios from permafrost explored by the paper include both methane and carbon. Here's what the paper says:

"The most important determinant of whether release of frozen carbon happens as CO2 or CH4 [methane] is whether decomposition proceeds aerobically or anaerobically... In anaerobic conditions, a greater proportion of soil organic carbon decomposition is released as CH4, although not all of it necessarily reaches the atmosphere."

Following this paragraph, the paper cites several scenarios for large-scale releases from permafrost carbon, including the 50-100 Gt carbon release I mentioned.

Further down, the paper continues:

"Thawing of the terrestrial permafrost will result in CO2 and CH4 emissions on time scales of a few decades to several centuries."

So Tobis is wrong in assuming that the carbon release scenarios the paper is discussing are only CO2 - that isn't specified, so I'd assumed the paper was open on whether the 50-100 Gt emissions were methane or carbon.

This was a mistake, however. The paper makes clear that although the scenarios are not clear on the precise quantification of carbon dioxide compared to methane releases from permafrost thawing, methane releases would be only be a small percentage of the overall carbon release scenarios explored. So Tobis is ultimately correct - the paper does not back up the specific scenario endorsed as likely by the Nature paper. I stand corrected on that.

Therefore, the plausibility of the specific 50 Gt scenario rises and falls on the credibility of the four Arctic specialists, including Dr. Natalie Shakhova, who came up with the scenario in the first place. That leaves point 1 only half intact, so we're left with:

Tobis unfortunately addresses this with only an ad hominem attack on the expertise of these Arctic specialists:

"Whether we should be acknowledging the 'Arctic specialists' as actually expert is, frankly, the question at hand."

Tobis goes through my other citations of the literature arguing that I am confusing quantities and making unwarranted extrapolations. However, my citations of this literature is simply to clarify that the literature does not rule out potentially dangerous releases of Arctic methane. Does Tobis manage to refute point 2. Arctic methane hydrates are becoming increasingly unstable in the context of anthropogenic climate change and it's impact on diminishing sea ice? No. Arctic methane hydrates are becoming increasingly unstable. I said nothing more, or less, than exactly that.

What about fact 3. Multiple scientific reviews, including one by over 20 Arctic specialists, confirm decadal catastrophic Arctic methane release is plausible?

Tobis concedes "A couple of reviews do give some support to this, but are vague about time scales". He then links to what he describes as a "DOE report". Instead, the link goes through to a Geophysical Research Letters study, which, however, he completely ignores, instead quoting from the original Review of Geophysics paper as follows: "The risk of a rapid increase in [methane] emissions is real but remains largely unquantified..."

And he calls me confused!

He then argues that there is "plenty of room for acceleration without hitting the cataclysmic level. Further evidence doesn't support the immediacy of that scenario at all."

But the Review of Geophysics paper does NOT say that there is "plenty of room for acceleration without hitting the cataclysmic level" - it says that:

The paper does NOT say available evidence "doesn't support the immediacy" of a catastrophic scenario, but rather that "uncertainties are large, and it is difficult to be conclusive about the time scales and magnitudes of methane feedbacks."

"... while many deep hydrate deposits are indeed stable under the influence of rapid seafloor temperature variations, shallow deposits, such as those found in arctic regions or in the Gulf of Mexico, can undergo rapid dissociation and produce significant carbon fluxes over a period of decades."

I think my fundamental contention - that the scientific literature recognises the possibility of some sort of catastrophic methane scenario - remains valid. Tobis is right, however, to emphasise that there is very little evidence available on quantifying that possibility.

In response to fact 4. Current methane levels are unprecedented, Tobis says yes, but they are "not climbing rapidly", and therefore this is mere "hype." My intention here was not to suggest that current Arctic methane levels are definitive evidence of a catastrophe already underway, but simply to note that it is wrong to say methane levels are NOT rising. They are, and once again, Arctic specialists are concerned.

"The CARVE science team is busy analyzing data from its first full year of science flights. What they're finding, Miller said, is both amazing and potentially troubling.

'Some of the methane and carbon dioxide concentrations we've measured have been large, and we're seeing very different patterns from what models suggest," Miller said. "We saw large, regional-scale episodic bursts of higher-than-normal carbon dioxide and methane in interior Alaska and across the North Slope during the spring thaw, and they lasted until after the fall refreeze. To cite another example, in July 2012 we saw methane levels over swamps in the Innoko Wilderness that were 650 parts per billion higher than normal background levels. That's similar to what you might find in a large city.'

"Ultimately, the scientists hope their observations will indicate whether an irreversible permafrost tipping point may be near at hand. While scientists don't yet believe the Arctic has reached that tipping point, no one knows for sure. 'We hope CARVE may be able to find that "smoking gun," if one exists,' Miller said."

So while NASA Arctic specialists say Arctic methane levels are "amazing" and "potentially troubling", outside the range of most model predictions, and possibly indicative that "an irreversible permafrost tipping point" is near - a matter which "no one knows for sure" - Tobis wants to interpret all the evidence as "refuting" any need for concern.

The other problem is that Arctic monitoring is still poor, and might be missing significant methane emissions. As Shakhova and her co-author Igor Semiletov told the New York Times' Andy Revkin:

"It is no surprise to us that others monitoring global methane have not found a signal from the Siberian Arctic or increase in global emissions... The number of stations monitoring atmospheric methane concentrations worldwide is very few. In the Arctic there are only three such stations - Barrow, Alert, Zeppelin - and all are far away from the Siberian Arctic. We are doing our multi-year observations, including year-round monitoring, in proximity to the source. In addition to measuring the amount of methane emitted from the area, we are trying to find out whether there is anything specific about those emissions that could distinguish them from other sources. It is incorrect to say that anyone is able to trace that signal yet."

Most Arctic specialists recognise that there's simply not enough research to justify dismissing the possibility of a catastrophe. That sword cuts both ways, of course - equally, there's not enough research justifying conclusions that we are definitely on the brink of a catastrophe.

On 5. The tipping point for continuous Siberian permafrost thaw could be as low as 1.5C, Tobis concedes this "is on the table", but that "it has nothing to do with undersea methane." Um, I never said it had anything to do with undersea methane.

On 6. Arctic conditions during the Eemian interglacial lasting from 130,000 to 115,000 years ago are a terrible analogy for today's Arctic, he writes: "as a response to Chris Colose" this is a "terrible" response, "because Colose is not relying on the Eemian but on the early Holocene as the analogous period." Yes, Colose does refer to the early Holocene, but he also repeatedly refers to the Eemian, the "Last Interglacial period between 130,000 to 120,000 years ago." In a previous article, I'd already mentioned that in the early Holocene, the East Siberia Arctic Shelf (ESAS) was "not an underwater shelf but a frozen landmass" as reason to be sceptical that paleoclimate data provides a ready analogue for the present.

Tobis then launches an ad hominem attack on climate scientist Paul Beckwith, whom I quoted for this article, and whom Tobis refers to as:

"'Prof' Paul Beckwith, the 'Professor Beckwith' who is a grad student at Ottawa U."

For the record, earlier this year, Beckwith formally passed his PhD examination on abrupt Arctic climate change at the Laboratory for Paleoclimatology and Climatology, University of Ottawa, where he is currently a part-time professor in climatology. Rather than addressing Prof Beckwith's argument, Tobis wants to demean his reputation and ignore his argument (which he fails to refute). Beckwith's full response to Colose is here. Among Beckwith's points, he argues that neither the early Holocene nor Eemian offer good analogues for the present Arctic:

"Earth tilt was larger, so Winter Northern Hemispheric solar radiation was about 40 W/m2 lower than today at 60 degrees North. Thus, the ice formed much more quickly and much thicker in the winter back then. Also, at night much more heat was radiated out to space in the lower GHG world then as compared to our 400 ppm levels today... the summertime Arctic is not believed to be seasonally ice free during these periods. The last time this happened was likely 2 or 3 million years ago... Colder winters in the early Holocene and Last Interglacial and much colder nights (in summers and winters then) meant much thicker and extensive ice formation in winters, and slower melting at night, respectively."

If I was to take Tobis' approach, I could have noted that Chris Colose is a "grad student" at the University of Albany. I didn't, because it's irrelevant.

Finally, Tobis takes on fact 7.Paleoclimate records will not necessarily capture a large, abrupt methane pulse with the following obfuscation: "Now, we swing back to saying that it HAS occurred in the recent geological past, indeed at the time which Colose says is the better analogy."

This is incorrect. Here, I merely point to a paper in Science by Nisbet which argues specifically that the cold Younger Dryas was ended due to methane emissions which came mostly from wetlands, but for which the initial trigger could have been Arctic methane clathrates:

"A possible explanation for the sudden end of the Younger Dryas is that, at a time of high Arctic insolation, an initial outburst of methane - perhaps from a geological source such as methane clathrates - triggered global warming, initiating both strong wetland emission in the tropics and north (8), and further hydrate responses as the thermal shock penetrated the permafrost (9, 10), freeing methane from decomposing clathrate hydrates and releasing gas pools trapped beneath them."

The evidence for this, however, is inconclusive, so the paper concludes: "The jury thus remains out on the initial trigger..."

On the issue of whether paleoclimate records will actually capture a large, abrupt methane pulse such as the scenario proposed by Shakhova et. al, as this paper in Earth and Planetary Science Letters observes, "rapid methane perturbations in the atmosphere are strongly smoothed in ice core records" due to "the relatively short atmospheric lifetime of methane." So it is quite possible that an abrupt, catastrophic methane release of the sort Shakhova proposes has happened, but is undetected in ice cores.

Tobis then declares a "scientific consensus has been reached" that Shakhova's scenario is "implausible in the extreme."

But the scientific consensus amongst ESAS experts is quite different, as I'd already noted. A peer-reviewed study by 20 Arctic specialists of ESAS data from 1995-2011, drawing of course also on Shakhova's work, specifically recognises:

"The emission of methane in several areas of the [ESAS] is massive to the extent that growth in the methane concentrations in the atmosphere to values capable of causing a considerable and even catastrophic warning on the Earth is possible."

It seems clear to me that the scientific literature on the danger of an Arctic methane catastrophe recognises the possibility unequivocally, but highlights huge uncertainty in our knowledge of the processes at work. Most of the literature I've been able to find on this subject shows great humility - and while acknowledging the possibility of worst-case scenarios, makes quite clear that the likelihood of those scenarios is very difficult to gauge.

Tobis also refers to a response to the Whiteman paper submitted to Nature (though not yet published) by Nisbet et. al, which argues that Shakhova's scenario is "improbably large" as there is no evidence for such events during past "glacial/postglacial transitions."

This is certainly a notable contribution to the debate, but if past paleoclimate conditions are not a good analogue for present Arctic conditions - a matter which remains a matter of scientific debate - and if ice cores would not record such a rapid scenario, then the central argument of this paper may be questionable.

"... the rarity of palaeoclimate evidence for hydrate-induced climate changes argues that this is a fairly unlikely candidate for near-term sudden climate change. Unlike the others, however, anthropogenic climate change may alter the probability of hydrate release when compared with the past, making the overall probability of near-term release extremely difficult to estimate...

Massive methane release by hydrates or from peats also seems to have been extremely rare in the past, but could become more probable in the future world under the influence of anthropogenic forcing. However, at present, it is not possible to judge the probability for such changes reliably."

Shindell's argument offers a warning that lack of past evidence is not a reason for present complacency where anthropogenic forces are changing the climate in ways not necessarily captured by paleoclimate evidence. So where does this leave us with regard to the risk of abrupt, catastrophic methane releases? As far as I can discern, the literature is largely agnostic about it, emphasises that specific scenarios are difficult to quantify, and calls for further research. The Review of Geophysics paper, for instance, far from asserting that a catastrophic methane release is refuted by geophysical evidence - as Tobis says - concludes:

"A significant increase in CH4 emissions and atmospheric concentrations due to climate change is therefore a possible scenario for the next century. However, uncertainties are very large, and as discussed above, it is difficult to be very conclusive regarding the magnitude of CH4 feedbacks and their time scales."

What about Shakhova et. al's specific scenario of a potential 50 Gt methane release at any time (the basic contours of her argument are outlined here, no paywall)? Shakhova et. al say simply that the scenario should be taken seriously as a possibility underscoring the importance of further ESAS research. The fact that Nature co-author Prof Peter Wadhams, who heads up polar ocean physics at Cambridge, also takes it seriously, is significant. Is Prof Wadhams' expertise also to be attacked? Ultimately, in my view, Tobis fails to show either that this scenario specifically, or abrupt methane catastrophe more generally, are unlikely.

In particular, his claim that there is a scientific consensus demonstrating near impossibility of a risk of a catastrophic methane event strikes me as unsupportable. Disagreement among scientists over the Arctic methane question is real, and it seems clear that Arctic specialists - Shakhova included - largely agree that while catastrophe is possible, more research is needed to discern how likely or unlikely it might be.

While other scientists, many reputable, argue importantly that such scenarios are beyond the pale, to my mind Tobis' egregious ad hominems against Arctic scientists whom he disagrees with have no place in scientific debate.